The invention relates to a device and method for doubling the frequency of electromagnetic radiation of a given frequency.
It is well known that non-linear optical effects can be produced in crystals (for example LiNbO.sub.3, .alpha.-Quartz, Tourmaline) for which the second order non-linear dielectric susceptibility is not zero, that is birefringent crystals which do not have a centre of inversion symmetry. Such crystals exhibit, under certain conditions, phenomena such as sum and difference frequency generation, second harmonic generation (frequency doubling) and parametric amplification and oscillation of electromagnetic radiation input to the crystal. In order to obtain frequency doubling or second harmonic generation using such non-linear crystals, it is necessary to ensure phase-matching between the input electromagnetic radiation and the frequency doubled electromagnetic radiation. Thus the refractive indices at the input frequency and the doubled frequency in the crystal must be equal. Accordingly to ensure phase-matching, a direction at an angle to the axis or axes of symmetry of the birefringent crystal must be selected at which the refractive index at the input frequency for the ordinary ray is equal to the refractive index at double the input frequency for the extraordinary ray so that when electromagnetic radiation at the desired input frequency is input to the crystal at that selected angle to the axis or axes of symmetry, an extraordinary ray at twice the input frequency is generated along the same direction as the input ordinary ray. Accordingly, it will be seen that precise orientation of the non linear crystal is needed to obtain the required phase-matching. Moreover, the frequency doubling process using such non-linear crystals is not particularly efficient and a powerful laser input is normally required to obtain good results.